1. Technical Field of the Invention
The present invention relates to a manufacturing method of piezoelectric ceramic materials useful for a ceramic resonator, a ceramic filter, a surface acoustic wave element, a piezoelectric displacement element, an ultrasonic transducer or the like, and more particularly to a manufacturing method of piezoelectric ceramic material which is preferred for piezoelectric element for high frequency use, utilizing third harmonic wave of thickness longitudinal vibration and high in machine quality coefficient.
2. Related Art
In recent years, piezoelectric element is widely used in the ceramic resonator, the ceramic filter, the surface acoustic wave element, the piezoelectric displacement element, the ultrasonic transducer, etc. and the range of its use is more and more increasing. On the other hand, the required characteristics includes many things and becomes most severe.
In recent years, especially with the advancement of more higher frequency use of the piezoelectric element, the material which is excellent not only in piezoelectric characteristics but also in dielectric characteristics is in strong demand. For example, as the material for the surface acoustic wave element, strongly desired is the material which is large in electromechanical coupling coefficient and small in temperature coefficient of surface wave speed (or delay time) and moreover which has characteristics, wherein dielectric constant is small because of use in high frequency of 10 MHz or more.
Now, as the piezoelectric materials for the surface acoustic wave element, LiNbO.sub.3 single crystal, LiTaO.sub.3 single crystal, ZnO thin film, piezoelectric ceramics, etc. are known so far, among which the piezoelectric ceramics are advantageous from a cost point of view and characteristics thereof can be diversified by changing the composition. Particularly, among those materials, PZT group materials have been vigorously developed so far, which are preferred in piezoelectric characteristics and temperature characteristics.
As the PZT group materials, those materials which have lead zirconate titanate as its main component and are added with metallic oxides such as Mn, Cr, Co, Fe, etc. or those materials which make the composite oxides as represented by Pb (Mg.sub.1/3 Nb.sub.2/3)O.sub.3 into solid solution and various physical properties thereof improved are known. However, these materials are not suitable for the piezoelectric element for high frequency because dielectric constant tends to become high if temperature characteristics is improved in general. Therefore, various studies are being conducted on the composition group having a smaller dielectric constant as surface wave element. Yet, no materials have been found so far wherein effective dielectric constant is not more than 350. In the conventional PZT materials, if they were used in high frequency range, significant disadvantages followed where input-output impedance decreases and impedance mismatching is generated between the external circuit and them or a capacity coupling is established between the input and the output, etc., thereby causing ripple to the output waveform.
On the other hand, lead titanate group materials containing rare earth group materials and other metallic oxides at the same time are also proposed as the piezoelectric elements for high frequency. However, these materials are low in dielectric constant, comparing to the PZT group materials and, though good in piezoelectric characteristics, they are inferior in a sintering property and difficult to make large sized ceramics. Moreover, a polarization condition is severe and there is a disadvantage in that a yield in mass production is low.
As the materials which overcame the disadvantages of the conventional piezoelectric element materials and which have low dielectric constant and high mechanical quality coefficient (mechanical Q value: Qm value), those materials which have lead titanate, bismuth titanate and calcium titanate as the basic constitutional components but which are substituted titanium by manganese or niobium are proposed (Japanese Patent Application Kokai No. 191055/1985). However, in case where they are used as high frequency resonators utilizing third harmonic wave of thickness longitudinal vibration, there arise disadvantages in that excellent physical properties of the materials peculiar to these materials such as small temperature coefficient of resonance frequency (TCF) and high machine Q value can not be fully displayed.
In case where the piezoelectric elements are used as high frequency elements utilizing third harmonic wave of thickness longitudinal vibration, as an experiment to make temperature coefficient of resonance frequency small and mechanical Q value high, there are, for example, the piezoelectric ceramics which are described in Japanese Patent Application Kokai No. 58724/1993 and Japanese Patent Application No. 353674/1993.
However, these piezoelectric elements are unable to correspond to the trend toward miniaturization and low-voltage driving. That is to say, for example, in case of ceramics oscillator for hard-disc driving, there is a need to realize frequency more than 40 MHz in a shape not more than 40 mm square and yet it is difficult for the piezoelectric ceramics with lead titanate as a main component as described in said Japanese Patent Application Kokai No. 58724/1993 or Japanese Patent Application No. 353674/1993 to correspond to such a need since mechanical quality coefficient (Qm) is not good enough in high frequency range.